Quick question, I'm investigationg ablative liquid rocket engines and want to know what would make a good ablative liner for the combustion chamber? Ceramic would be my first choice but after hunting down some, the price just kills it, secondly since I've got a chank of graphite for the nozzle so I thought I could go ahead and make the whole chamber/nozzle from graphite, but I believe its susceptible to cracking (correct me if I'm wrong) and although not as expensive as ceramic for me to get I wouldn't want to be replacing a new chamber every month or so during testing.Everything is to be mounted in a SS outer case, so bear this in mind.Or could the graphite chamber be inclosed in the ss tube? or does it expand too much with the heat?This particular engine only has a burn time of 10sec.Composites are out of the question as well, too expensive.

Well, for cheap I would go with graphite, it is easy to get a hold of and use, ceramics are much more sophisticated from my limited experience. I would just line the inside of the chamber with the graphite that you have. I do not believe that the graphite expands much, but you mentioned cracking as a concern. I think that it will be fine since you said the rocket engine is only running for 10 seconds.

This is the voice of naive ignorance speaking here, but why are composites unreasonably expensive? What sort of composites are we talking about? If their job is to be ablated, what particular properties do they need? (See, I wasn't kidding when I said 'naive ignorance'!)

We tried phenolic in our solid fuel combustion chamber and let's just say, it didn't work out. After numerous static motor tests, I can say our ablative coating we put on our steel nozzle and aluminum bulkheads works fine when our various combustion chamber materials has failed. We decided to try making our propellant casting tubes out of the same stuff and seems to work fine (subjected to heat for 20 or so seconds). It has a similar formulation of Apollo Avcoat 5026 (heat shield).

I've been wondering about machined graphite (cheap, good ablative properties, but brittle) reinforced with home-made carbon-carbon composite (carbon fiber in a carbon matrix built by heating heavy oil in an inert atmosphere such as argon or vacuum). What do you all think?